1. Field of the Invention
The invention relates generally to testing of circuit assemblies, and more particularly to capacitive testing of connections to pins of components of the circuit assembly.
2. Description of Related Art
During the manufacture of printed circuit board (PCB) assemblies, testing is performed at one or more stages to ensure that the finished product functions adequately. In some manufacturing operations, circuit assemblies are first tested before any components are mounted to them. The components may be separately tested before they are attached to the circuit assemblies, usually by soldering. Once the components are attached, a further test may be performed to verify that the components are properly attached. Such testing includes “opens” tests and “shorts” tests that indicate defects in attachment of contact points of the components to the circuit assembly. These contact points are generally referred to as “pins,” even though the contact points may take many shapes, including posts, gull wing leads or solder balls.
One approach to testing the connections to pins is called capacitive testing. In a capacitive “opens” test, a probe that includes a sense plate is pressed against a component with a pin to be tested. A test signal is generated at a point on the circuit assembly that should be connected to the pin under test in a properly manufactured circuit assembly. If the pin under test is properly connected to the circuit assembly, the test signal will propagate from the circuit assembly through the pin to conducting structures within the component. Though the probe plate is separated from those conducting structures, the test signal can capacitively couple to the probe plate. During a test, the signal received at the probe plate is analyzed to determine whether the test signal has been capacitively coupled to the probe plate, indicating that there is a good connection between the assembly and the pin of the component.
Though capacitive testing may be desirable, it has not been widely used for certain types of components. For example, sockets have not been widely tested using capacitive test techniques. Sockets generally contain a cavity shaped to receive a semiconductor chip or other component. The pins of the socket extend into the cavity and provide spring contacts to which the other component can be connected. The socket can be soldered to the circuit assembly and the chip can later be inserted. When the socket is tested without the component inserted, there is little conductive structure in the socket through which a test signal applied to a pin under test can couple to a probe plate. Accordingly, even when a pin under test is properly connected to a circuit assembly, it is difficult to reliably detect a capacitively coupled test signal, increasing the likelihood of errors when testing pins of a socket capacitively.
In some instances, the probe has been inserted into the socket to increase the amount of the test signal coupled to the probe plate. However, inserting a probe into the cavity introduces a substantial risk of contacting and damaging the pins.
Also, it is known to include a guard plate in a probe for capacitive testing. The guard plate shields the sense plate from electrical noise that could change the signal detected at the sense plate enough to produce an erroneous test result.